As known in the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw are/is employed. In twin-wire draw, the groups of drying cylinders include two wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering which may result in web breaks, in particular since the web is still relatively moist and, therefore, has a low strength. For this reason, recently, increasing use has been made of the single-wire draw in which there is only one drying wire in each group of drying cylinders, the web running on the support of this wire through the entire group so that the drying wire presses the web on the drying cylinders against heated cylinder faces and the web remains at the side of the outside curve on the reversing cylinders or rolls arranged between the drying cylinders. Thus, in single-wire draw, the drying cylinders are situated outside the wire loop and the reversing cylinders or rolls are situated inside the wire loop. As is well known, in the dryer sections in paper machines, both dryer groups with single-wire draw and dryer groups with twin-wire draw are frequently employed.
The properties required from the drying wires, which are usually made of textile fabric, are different depending on the mode of operation concerned, i.e., single-wire or twin-wire draw.
Accordingly, in single-wire draw, the function of the wire is to support the paper web as a so-called "closed draw". Also, the wire supports the paper against the face of the drying cylinder. In order that the paper is kept in contact with the wire in single-wire draw, various grooved and/or suction rolls are often used as well as blow-suction boxes which produce a vacuum between the cylinders and rolls, such as the stabilizers marketed by the current assignee under the trademark "Uno-Run".
Owing to the above, a permeability within certain limits is required from the wire. When a new wire is acquired for use in the paper machine, it is possible to choose the permeability, but it is often problematic to make the permeability remain within certain limits, because the wire is contaminated during operation.
Since the mechanical service life of a wire is long (about one year), the wire is expensive, and the replacement of the wire always requires a standstill, various wire washing devices are used in order to maintain the permeability of the wire within the desired operating parameters.
In prior art applications, the washing devices are placed on a free, so-called returning portion of the wire, i.e., that portion which usually starts at an upward run of the drying wire after the last drying cylinder or reversing roll or guide roll to the first drying cylinder or reversing roll or guide roll at the beginning of the dryer group. For this reason, the washing liquid can spread over a wide area onto the machine, in which case a wide area can be contaminated and become wet by the effect of splashing, and the washing device can be employed at a very low running speed of the machine only.
Moreover, the prior art wire washing devices wash just a part of the wire width at a time. Also, frequently the spacing/oscillation of the nozzles in the washing device is so slow that the device would have to be operated in normal paper running, which is difficult to arrange because of the splashing of liquid.
In twin-wire draw, besides providing adequate support for the paper web on the cylinders, a function of the wire is to take care of the ventilation of the so-called pocket spaces, i.e., to carry away the evaporating liquid out of the closed space formed by the paper and the wire. As is the case also in single-wire draw, this function requires that the permeability of the wire is retained within certain operating parameters as long as possible. The problems of the prior art washing devices in connection with twin-wire draw are the same as in connection with single-wire draw applications. With respect to different devices for washing of drying wires and related arrangements, which are in themselves known, reference can be made, for example, to Finnish Patents No. 67593 and 60045, and U.S. Pat. No. 3,910,815 (incorporated by reference herein).